1. Field of Invention
The present invention relates to a compensation method and circuit to enable the pickup head position of a disk storage system to displace, and particularly to a method and a circuit for compensating slow-varying runout of a disk storage system.
2. Description of the Related Art
The so-called “disk storage system” means a data storage system using circle disks as data storage media, where circle tracks are available for storing data. To properly write data into the tracks or read data from the tracks, precisely positioning of the pickup head for reading and writing data is essential.
To have the pickup head precisely positioned, any unstable state during spin of a disk must be considered. As shown in FIG. 1, a rotatory motor 100 is used for driving a disk 110 to spin, a pickup head 120 can vertically and laterally move by means of a positioning server 130 for aligning the data tracks. However, a disk runout during the spin thereof is likely to happen due to the disk error itself or a design error of the magnetic disk drive. Referring to FIG. 2A, a diagram shows a runout of a disk storage system during the spin thereof. The center of a disk 200 locates at a position 210 shown in FIG. 2A and circle data tracks 220 are distributed around the disk center. When the disk 200 is placed in the disk drive for writing or reading data, due to a non-coincidence between the turning center and the disk center, the disk 200 may spin around the turning center 215, instead of the desired disk center. Therefore, during the spin the data tracks 220 will displace, for example, taking a position of the dot-line circle 225 at a time shown in FIG. 2A. In other words, the running paths of the data track 220 are more like a hula hoop, rather than a desired and fixed circle. Another kind of runout is an axial runout as shown in FIG. 2B. Different from the above-described radial runout, the axial runout is due to a warp of the disk 200, which causes the disk to vibrate up and down during spinning.
To resolve the above-described runout problem, several resolutions have been provided. U.S. Pat. No. 6,088,186 provides a feedback control system and a circuit with a high-gain at the runout frequency to compensate the runout error. We note that the scheme is valid only in a condition with a stable rotate speed of the disk; hence, it works for a magnetic hard disk drive only where rotate speed of the disk is relatively stable, but not for an optical disk drive or a magnetic-optical disk drive.
In addition, a so-called “feed-forward control system” for compensating the runout is provided by, for example, U.S. Pat. Nos. 6,831,803, 6,549,362 and 6,545,835. According to the scheme provided by those patents, the errors caused by the online runouts are recorded, which are taken out for the disk storage system to compensate the runouts at some certain different spin positions. It can be seen that the scheme is suitable for those disk storage systems which do not require changing disks frequently, such as a magnetic hard disk drive. For a disk storage system requiring frequent changes of disks, for example, an optical disk drive, however, the scheme hereinabove is not adoptable. Since each disk has different runout and the recorded error corresponding to the runout of the last disk just drawn out is not the same as the present disk just put in the tray of an optical disk drive, the recorded runout control is not workable in this aspect.